Method of quantitatively evaluating roughness of metals



Patented June 25, i946 Fi/T ETHOD F QUANTRTATHVELY EVALUAT- ENG ROUGMESS 0F METALS Harry K. Herschman, Washington, D. C.

application November 25, 1944, Serial No. 565173 (Cl. SS-li (Granted under the act of March 3, 11883, as amended April 30, i928; 370 @l G. 57)

3 Claims.

The-invention described herein may be manufactured and used by or for the Government of the United States for governmental purposes without the payment to me of any royalty thereon in accordance with the provisions of the act of April 30, 1928 (Ch. 460, 45 Stat. L. 467).

My invention provides a new and useful method for quantitatively evaluating the roughness of surfaces and particularly for finished surfaces of metals.

My method utilizes and preserves uninjured la. A record of a surface finish in the form of a plastic facsimile.

My facsimiles may be readily forwarded to the measuring equipment, thus avoiding any need for transporting either such equipment or the article being measured.

The methods disclosed .herein makes possible the evaluation of the surface nish of parts in a very brief period of time which parts may not be readily accessible with the presently available means of evaluating surface finish.

My method may also be used for measuring the finish of non-metallic materials and for the evaluation of surface corrosion, pitting and pinholing of metals. f

The personal equation and error and the necessity for the services of highly skilled personnel are eliminated by my method.

Most of the contrivances which have been developed and used for quantitatively evaluating the roughness of nished surfaces of metals are time-consuming and limited in their practical applicability. Some of the commonly used devices employ a stylus to determine the roughness of a surface, but only a very restricted area can be tested in one setting of such instruments. A stylus for this use is commonly made of a very hard material, such as diamond, which frequently scores the surface being measured, especially metals of relatively low hardness values such as annealed steels or bearing alloys, and in addition, the tracer point of the stylus may fracture or become deformed by Wear, necessitating replacement. The method which I have developed entails none of the objections mentioned above.

Essentially, my method is basedson the use of a lm replica of a surface which is a reproduction in the minutest detail of the protuberances and recesses of such surface. 'Ihe accuracy of such reproduction is attested by the fact that a plastic surface replica is used in the study of microstructures at high magnications by the electron microscope.

The replica used is prepared by applying to a surface to be evaluated a suitable plastic either as a liquid or as a liquid in combination with a preformed transparent plastic lm with which it unites and, after drying, the replica is stripped from the surface, whereby every recess and protuberance is represented, respectively, by a corresponding protuberance or recess on the replica.

The evaluation consists essentially in passing a beam of light through such a replica onto a photoelectric cell and simultaneously moving the replica with respect to the light beam or the light beam with respect to the replica. The movement of the light beam with respect to the replica or of the replica with respect to the light beam may be either of straight line or curved motion, the path of motion being maintained in a manner such as to cause such path to cross the protuberances and recesses of a surface or portion thereof. Even very minute variations in the number and nature of the protuberances and recesses on the replica film produce corresponding variations in the light transmitted by the replica and falling upon the photo-electric cell. This necessarily produces a correspondingly varying current, which can be measured by any suitable electronic volt-meter or the like. These current readings or records are then translated into roughness evaluations of the surface by means of a calibration of the device obtained with surfaces, the roughness of which is determined by microscopic means.

In the accompanying diagrammatic drawing there is illustrated an apparatus suitable for use with my novel method.

Figure 1 diagrammatically illustrates this apparatus.

Figure 2 diagrammatically illustrates a detail of a modification.

Figure 3 diagrammatically illustrates a greatly magnified section of a facsimile. f

In these diagrams, I I indicates a suitable source of light, such as an incandescent bulb, projecting a pencil-like beam of substantially parallel rays I2, upon one of my facsimiles I3 mounted in a frame which is reciprocated at a suitable rate by a suitable motor (not shown) and a suitable means for at will varying the speed of reciprocation, as by a lever-controlled eccentric. 'Ihe pencil of light is transmitted through the reciprocating facsimile and impinges upon a photo-electric cell I4 which causes an electric current supplied by any suitable source such as a dry battery or other source of current I6 to pass through an electric circuit I5 including a switch I9. The light transmitted by the reciprocated facsimile I3 is variable because of differences in the geometric characteristics oi' the facsimile I3 from area to area scanned by the pencil of light as illustrated in Figure 3. These uctuatlons of the transmitted light cause corresponding :ductuations of the photoelectric current in the electric circuit I5. Resistance 20 is constant, and therefore the voltage drop across resistance 2n varies proportionately to the photoelectric current. 'Ihis varying voltage is impressed on a suitable electronic voit-meter I'I which. being an alternating-current measuring device, gives a reading which is related to the variations of the voltage and hence to the variations of the photoelectric current. The variation of photoelectric current which is related to the degree of variation of the light transmitted by the reciprocated facsimile I3 increases as the variations in the number and nature of the protuberances and valleys in diiIerent areas on the facsimile become more pronounced or in other words, as the degree of roughness of the reproduced surface increases.

An'alternative mounting support for the facsimile is diagrammatically indicated in Figure 2 in which 22 indicates asuitable drum, in lieu of the vertically actuated frame indicated in Figure 1, and suitable means, not shown, rotates or oscillates the drum and facsimile mounted thereon and 23 indicates a photoelectric cell mounted within the drum 22,'but supported independently of the drum. The photoelectric cell I 4 is enclosed by a shield 24 made of a light-proof material, and provided with a suitable slit 25 which permits a restricted portion of the light transmitted through the facsimile I3 to affect the photoelectric cell Il.

My method does not wear away the surface of the replicas used, which may be led away for future use or comparisons and may be easily sent by mail to a central testing laboratory from distant places where the essential equipment is not available.

It should be understood that my invention includes all modiiications and equivalents which fall within the present disclosure and within the scope of the appended claims.

What I claim is:

1. The improvement inthe art of comparing 4 the roughness of finished surfaces from transparent replicas o1' said surfaces, which comprises directing light through a transparent surface replica upon a photoelectrically responsive means while effecting a movement of said replica with respect to said responsive means and reading the average of the variations of the instantaneous values of the photoelectric current :between a minimum and a maximum value, such variations being a function of the contour of the replica and hence of the roughness of the surface, and comparing such reading with a reading obtained in the same manner from a. replica of another surace.

2. The improvement in the art of comparing the roughness of nished surfaces from transparent-replicas of said surfaces, which comprises directing a pencil of light through such a replica upon a photoelectrlc cell upon which a. suitable y voltage is impressed, moving said replica with respect to said cell to cause said pencil of light to scan a selected portion of said replica and reading the average of the variations of photoelcctric current corresponding to the iluctuations of lnstantaneous values of light-received by said cell and comparing such reading with a corresponding reading from a replica of a surface'of a known degree of roughness.

3. The improvement in the art of determining the relative roughness of a surface by comparing a measurement of a transparent surface replica of a surface of a. known degree of roughness with that of a transparent replica of a surface to be measured which comprises directing a pencil of light through the replica of the surface of a known degree of roughness upon photoelectrically responsive means while moving said replica with respect to said responsive means, reading the average oi' the variations of the instantaneous values of the light received by said cell making a corresponding reading of a replica of a surface to be measured and comparing said reading of said replica of the surface to be measured with a that of said replica of known surface roughness.

HARRY K. HERSCHMAN. 

